Many structures, such as buildings, and systems, such as aircraft, contain some type of smoke or fire detection system that detects smoke or fire, and thereafter provides an indication that a fire may exist within the structure or system. In many structures or systems, such smoke or fire detection systems are installed within enclosures that include some type of ventilation that provides airflow through the enclosure. In the context of aircraft, for example, cargo or baggage compartments are provided with ventilation to control the temperature and air quality within the compartment. Such ventilation in enclosures, however, significantly impacts the ability of smoke or fire detection systems to detect smoke or fire.
Typically, smoke or fire detectors are arranged in one of two manners. As shown in FIG. 1, one or more spot detectors can be arranged in the enclosure, such as on the ceiling. Alternatively, an aspirated system (not shown) includes ducts that draw air from one or more locations to a central detector. Thus, as shown in FIG. 1, when a fire 10 starts in an unventilated enclosure 12, the fire typically produces a plume 14 of smoke that rises to the ceiling of the enclosure, spreads out in a relatively strong concentration and fills the enclosure from the ceiling down to the floor. As the plume spreads out in a relatively strong concentration, smoke detectors 16 can easily detect the smoke such that the system can thereafter report a fire within the enclosure.
Referring now to FIG. 2, in contrast to unventilated enclosures, ventilated enclosures 17 generally have at least one air inlet 18 whereby air enters the enclosure, and at least one air outlet 20 whereby air exists the enclosure. When a fire 10 starts in a ventilated enclosure, then, the plume 14 of smoke does not rise predictably as in an unventilated enclosure 12. Instead, the plume of smoke is disrupted and diluted by the flow of air through the enclosure, where movement of smoke is dominated by the airflow patterns. As such, unless the ventilation carries the smoke directly to one of the detectors 16, more time is required for the smoke to reach the detectors in sufficient concentration to trip an alarm, as compared to instances of fires in unventilated enclosures. Furthermore, in instances in which a small fire occurs in the enclosure, the ventilation may prevent detection of the fire altogether. In this regard, if the fire is small enough, ventilation may cause the smoke plume concentration in the enclosure to stop increasing when the quantity of the smoke plume exhausted from the enclosure via the air outlet is equal to the quantity generated by the fire. As a result, the smoke plume concentration may not reach an alarm concentration, thereby allowing the small fire to propagate undetected.
Putting further constraints on performance of smoke or fire detection systems is that fact that many regulatory authorities place limits on the amount of smoke allowed to exist in a structure or system before being detected by an appropriate smoke or fire detection system. In the context of aircraft, for example, the Federal Aviation Administration (FAA) has imposed limits on the amount of smoke allowed to exist undetected in many portions of aircraft. In addition, the FAA over time has reduced limits on the amount of time allowed for a smoke or fire detection system to detect a fire in many portions of aircraft. Currently, for example, in cargo or baggage compartments, FAA Federal Aviation Regulation (FAR) 25.858(a) requires any certified smoke or fire detection system to provide a visual indication to aircraft flight crew within one minute after the start of a fire within the cargo or baggage compartments.
Conventionally, improving of detection performance of smoke or fire detection systems requires increasing the number of smoke or fire detectors, reducing the ventilation in the affected areas of the aircraft and/or increasing the sensitivity of the smoke or fire detectors. And whereas each technique for improving detection performance of smoke or fire detection systems is adequate, each has drawbacks. Increasing the number of fire detectors, for example, increases system costs associated with new detectors, as well as new electrical power sources, wiring, flight deck messages, plumbing complexity, and cargo liner and structural interfaces. Reducing ventilation generally results in financial losses to the aircraft operator in that to reduce the ventilation, the quantity of some cargo types must typically be reduced, thus reducing the capacity of the affected area and the overall aircraft.
While increasing the sensitivity of the smoke or fire detectors will increase system performance, the number of false alarms initiated by the smoke or fire detectors will also increase. In this regard, the frequency of false alarms is often considered one of the biggest problems with conventional smoke or fire detection systems. Increasing false alarms, in turn, decreases system reliability and can impose considerable costs for the aircraft operator and can result in unnecessary bodily injury to passengers, as described below.
False alarms can be generated when nuisance sources such as dust, moisture, and/or gasses, are presented to a detector at a level exceeding the alarm threshold. And whenever a fire alarm is triggered on an aircraft, for example, the aircraft crew typically discharge fire extinguishers in the affected area, divert the aircraft to the nearest airport, and occasionally initiate an emergency evacuation of the aircraft. By increasing the number of false alarms, the airlines incur costs associated with replacing expended fire extinguishers, accommodating inconvenienced passengers and dispatching the aircraft from an unplanned destination. In addition, unnecessary emergency evacuations can result in unnecessary passenger injuries, which can occur during emergency evacuations.
Although the foregoing has described systems and structures as including smoke or fire detection systems, it should be appreciated that such systems and/or structures can additionally or alternatively include detectors for detecting other types of hazardous conditions. For example, such systems and/or structures can include detection systems for detecting certain gases, such as carbon monoxide, that can be every bit as dangerous as fire or smoke (caused by fire). Thus, it should also be appreciated that such detection can have the same type of drawbacks as smoke or fire detection systems, described above.